1. Field of the Invention
The present invention relates to a display device and a method of driving a display panel. In particular, the present invention relates to a technique for desirably adjusting gray-scale on the display panel by performing a correction to a gray-scale data.
2. Description of Related Art
In recent years, a mobile terminal such as a mobile phone or a PDA (Personal Data Assistant) has been required to support a function of displaying movie. For example, a mobile phone supporting the digital terrestrial broadcasting is one of key products for a manufacturer of the mobile phone.
One problem is that a small LCD (Liquid Crystal Display) device of the mobile terminal is inferior in display quality of the movie, particularly in contrast characteristics at a time of when an image is not bright enough, as compared with a CRT (Cathode Ray Tube) or a big LCD device. In the LCD device of the mobile terminal, brightness of its back light is set low from a viewpoint of reduction of electric power consumption. As a result, when a movie is displayed, deterioration of picture quality is likely to occur due to insufficient contrast at the time when the image is not bright enough.
One method for improving display quality is to perform a correction operation, for example a gamma correction with respect to an input gray-scale data to enhance the contrast. Japanese Laid-Open Patent Application JP-H07-281633 (U.S. Pat. No. 3,201,449) discloses a technique to determine a gamma value depending on an APL (Average Picture Level) of the displayed image and variance (or standard deviation) of the brightness and to control the contrast by performing the gamma correction with the use of the determined gamma value. According to the technique described in the present patent document, when the gamma value is determined, a look-up table (LUT) in which input-output characteristics representing the gamma correction with the use of the determined gamma value are described is stored in a RAM. When an input gray-scale data is given, an output gray-scale data corresponding to the input gray-scale data is read out from the LUT, and thus the gamma correction is performed. Moreover, Japanese Laid-Open Patent Application JP-H09-80378 discloses a technique to perform a correction operation depending on the brightness of the back light and thereby to control the contrast of the image. According to the LCD device described in the present patent document, an LUT describing input-output characteristics with which a linear relationship between an input pixel data and an output pixel data can be obtained is prepared, and the correction operation is performed with the use of the LUT.
The inventors of the present application have recognized the following points. The LCD device performing the correction operation with respect to the image data is required to be small in its circuit size and low in electric power consumption. However, the LCD device performing the correction operation with the use of the LUT cannot meet such the requirement.
First, in the case of the LCD device performing the correction operation with the use of the LUT, it is necessary to prepare a high-capacity memory for storing the LUT, which causes increase in the circuit size. For example, in a case where the gamma correction is performed by using different gamma values for red (R), green (G) and blue (B), respectively, the input gray-scale data is of 6 bits and the output gray-scale data is of 8 bits, it is necessary to prepare an LUT whose size is 1536 bits (=26×8×3).
Furthermore, the LCD device performing the correction operation with the use of the LUT has a problem that the electric power consumption is large at a time when the relationship between the input gray-scale data and the output gray-scale curve in the correction operation is switched. That is, according to the LCD device performing the correction operation with the use of the LUT, it is necessary to rewrite the LUT in order to change the relationship between the input gray-scale data and the output gray-scale curve. However, a large amount of data transfer is necessary for rewriting the LUT. The large amount of data transfer causes increase in the electric power consumption, which is a problem particularly for the LCD device used in the mobile terminal.
As described above, in the display device configured to switch the relationship between the input gray-scale data and the output gray-scale curve in the correction operation depending on the image to be displayed, it is one important issue to achieve with a small circuit size and further to reduce the electric power consumption necessary for the switching.